S4, E5: Intelligence is tasked with tracking down the dealers of fentanyl a drug that has been linked to multiple overdoses around the city. As they investigate possible sources, the team uncovers a series of dealers in a college smuggling ring. Meanwhile, Tay (guest star Li Jun Li) and Burgess (Marina Squerciati) come across a bloodied child who exposes a dark and twisted secret about his home life with his parents. Additionally, Mouse (guest star Samuel Hunt) causes tension as he looks to reenlist in the army, and Platt (Amy Morton) tells Tay (guest star Li Jun Li) that she has been reassigned to her original post.